Contribution of actinorhizal shrubs to site fertility in a Northern California mixed pine forest

Bitterbrush (Purshia tridentata) and mahala mat (Ceanothus prostratus) are common N-fixing shrubs in interior forests of the western United States, yet their contribution to ecosystem N pools is poorly characterized. We compared N fixation and soil N accretion by these shrubs in old-growth ponderosa pine (Pinus ponderosa)–Jeffrey pine (Pinus jeffreyi) stands versus stands that had been harvested 50 years earlier. No differences (α = 0.10) in cover, biomass, or percent N derived from fixation by bitterbrush or mahala mat were found between harvested and uncut stands. Approximately 46% of bitterbrush N was derived from symbiotic N fixation as measured by the 15N natural abundance method. No accurate measure of percent N derived from fixation was attained for mahala mat using this technique due to the absence of a well-matched reference plant. Estimates of total N fixation rates in both stand types were 0.2 kg ha−1 year−1 for bitterbrush and 0.3 kg ha−1 year−1 or less for mahala mat. No appreciable soil N accretion resulted due to the presence of bitterbrush or mahala mat in either stand type. Nitrogen addition by these shrubs, although small, accounts for 10–60% of annual N input in these dry forest ecosystems.